1. Field of the Invention
The present invention relates to a focus detecting apparatus adapted for use in a photographic camera, a video camera, an observing apparatus or the like and an optical equipment utilizing such focus detecting apparatus, and more particularly to a focus detecting apparatus which is capable of dividing the pupil of an objective lens (phototaking lens) into plural areas, forming the distributions of optical amount on plural object images utilizing the light beams passing through such plural areas, and determining the relative positional relationship of such plural distributions of optical amount, thereby detecting the focus state of the objective lens for pluralareas, in two-dimensional or continuous manner, over a wide range within the image frame.
2. Related Background Art
Among the conventional passive focus detecting methods utilizing the light beam passing through an objective lens, there is already known so-called image aberration method (phase difference detection method).
FIG. 12 is a schematic view of the optical system (optical equipment) of a focus detecting apparatus utilizing such conventional image aberration method, wherein shown are an objective lens (phototaking lens) 101, a half-transmitting main mirror 102, a focusing screen 103, a pentagonal roof prism 104, an eyepiece lens 105, a sub mirror 106, a film (photosensitive plane) 107, and a focus detecting apparatus 108.
Referring to FIG. 12, a light beam coming from an unshown object is transmitted by the objective lens 101, then reflected upwards by the main mirror 102 and forms an image of the object on the focusing screen 103. The object image formed on the focusing screen 103 is reflected plural times by the pentagonal roof prism 105 and is viewed by an photographer or an observer through the eyepiece lens 105.
On the other hand, a part of the light beam reaching the main mirror 102 through the objective lens 101 is transmitted by a transmitting portion of the main mirror 102, then reflected downwards by the sub mirror 106 and is guided to the focus detecting apparatus 108.
FIG. 13 is a schematic developed view showing only the objective lens 101 and the focus detecting apparatus 108 shown in FIG. 12, for the purpose of explaining the principle of focus detection.
Referring to FIG. 13, in the focus detecting apparatus 108 there are shown a field mask 109 positioned in the vicinity of a plane conjugate with the anticipated focal plane of the objective lens 101 or the plane of the film; a field lens 110 positioned similarly in the vicinity of the anticipated focal plane; a secondary imaging system 111 consisting of two lenses 111-1, 111-2; a photoelectric converting elements 112 including two photosensor arrays 112-1, 112-2 positioned respectively corresponding to and behind the two lenses 111-1, 111-2; a diaphragm 113 having two apertures 113-1, 113-2 positioned respectively corresponding to the two lenses 111-1, 111-2; and an exit pupil 114 of the objective lens 101, including two divided areas 114-1, 114-2.
The field lens 110 serves to focus the apertures 113-1, 113-2 of the diaphragm 113 on or in the vicinity of the areas 114-1, 114-2 in the exit pupil 114 of the objective lens 101, whereby light beams 115-1, 115-2 passing through the areas 114-1, 114-2 of the exit pupil 114 respectively form the optical amount distributions of the object image on the two sensor arrays 112-1, 112-2.
The system of the focus detecting apparatus shown in FIG. 13 is generally called the phase difference detecting method (image aberration method). When the focus of the objective lens 101 is positioned in front of the anticipated focal plane, namely closer to the objective lens 101, the optical amount distributions of the object image, respectively formed on the two sensor arrays 112-1, 112-2, become positioned mutually closer. Inversely, when the focus of the objective lens 101 is positioned behind the anticipated focal plane, namely farther from the objective lens 101, the optical amount distributions of the object image, respectively formed on the two sensor arrays 112-1, 112-2, become mutually separated.
The amount of aberration of the two optical amount distributions of the object image, formed on the two sensor arrays 112-1, 112-2, is in a functional relationship with the amount of defocus of the objective lens 101, namely the focus. Thus such amount of aberration is calculated by suitable calculation means of detect the direction and the amount of defocus by using the focus of the objective lens 101.
In relation to such focus detecting apparatus, there have been contentionally made proposals for increasing the number of distance measuring points or expanding the viewing field, as disclosed for example in the Japanese Patent Laid-open Application No. 4-98236 and in the U.S. Pat. No. 4,147,417 and the like. These proposals are to achieve the expansion of the viewing field by disposing behind the main mirror of the single lens reflex camera, an optical member for convergingly reflecting the light beam transmitted by the objective lens.
The configuration disclosed in the Japanese Patent Laid-open Application No. 4-98236 is provided, behind the main mirror of the single lens reflex camera, a reflecting mirror which serves as a field mirror for forming two secondary object images by the two light beams passing through different specified areas of the pupil of the objective lens, whereby the focus state of the objective lens is detected based on the relative positional relationship of such secondary object images. However, the configuration of the Japanese Patent Laid-open Application No. 4-98236 has been associated with a drawback that the light inversely entering through the view finder system is also received by the focus detecting system as ghost light, since the light beam reflected by the field mirror is guided, after the light beam is reflected by about 90.degree. by the main mirror, to the bottom portion of the mirror box.
Also the configuration disclosed in the U.S. Pat. No. 4,147,417 is provided, similarly behind the main mirror of the single lens reflex camera, with a rear reflecting mirror having a converging power, for forming two secondary object images by the two light beams passing through different specified portions of the pupil of the objective lens.
The configurating in the U.S. Pat. No. 4,147,417 is principally different from the configuration of the Japanese Patent Laid-open Application No. 4-98236 in that the light reflected by the field mirror is directed in the diagonally front direction, through the main mirror. Such configuration allows to prevent the drawback of entry of the inversely entering light from the view finder system toward the focus detecting system.
On the other hand, the Japanese Patent Laid-open Application No. 63-243906 discloses a technical art for preventing the error in focus detection, owing to the color of the object, namely the wavelength of the light.
FIG. 14 is a schematic view of such technical art applied to the focus detecting apparatus shown in FIG. 13. In FIG. 14, there are shown a field lens 201, an infrared cut-off filter 202, a diaphragm 203 having a pair of apertures, and a re-imaging system 204 having a concave light incident surface and a pair of convex light exit surfaces. The light from the object, transmitted through an unrepresented objective lens, enters the field lens from the left in FIG. 14, and emerges from the re-imaging system 204 thereby forming a pair of secondary object images on a photoelectric converting element 205, which is supported in a sensor package 206. The light incident surface of the re-imaging system 204 is formed with a mild concave face to avoid steep refraction of the light entering the re-imaging lens 204, thereby suppressing the influence of the color aberration.
In the optical equipment incorporating the conventional focus detecting apparatus shown in FIG. 12, the light beam necessary for the focus detection is guided to the focus detecting apparatus 108 through the sub-mirror 106. Consequently the area allowing focus detection within the image frame is inevitably limited by the size (area) of the sub-mirror 106. Because of the positional relationship with the main mirror 102, it is difficult to expand the sub-mirror 106 particularly in the upward direction. It has therefore not been possible to expand the focus detecting area toward the upper part of the photographic film 107, or toward the lower part of the object field.
In the configuration shown in FIG. 12, in order to increase the area of the sub-mirror 106 without interference with the main mirror 102, it is conceivable to position the sub-mirror 106 more backwards. In such case, however, the anticipated focal plane of the objective lens 101, formed after reflection by the sub mirror 106, is displaced upwards so that the anticipated focal plane becomes separated from the focus detecting system 108 and the field lens (field lens 110 in case of FIG. 13) in the focus detecting apparatus 108 has to be made considerably larger. Such requirement constitutes a significant hindrance in disposing the focus detecting apparatus in the bottom portion of the camera.
In order not to increase the size of the field lens it is conceivable to displace the position of the field lens upwards corresponding to the upward displaced anticipated focal plane, but, in such arrangement, the field lens inevitably intercepts the phototaking light beam and has therefore to be retracted from the phototaking light beam at the phototaking operation. Such retraction required a very complex mechanical structure, which is not only costly but also cannot ensure the precision comparable to that in the conventional focus detecting apparatus.
On the other hand, the photoelectric converting element employed in the conventional focus detecting apparatus is principally a line sensor, so that the substantial area allowing focus state detection is limited to a very limited area, in fact a point or a line portion within the phototaking image area.
Also in the conventional technology there is already known a focus detecting apparatus having plural focus detecting systems and allowing focus detection in plural areas, but, in such device, the focus detecting areas are mutually isolated and the focusing operation is not possible for an object positioned between such focus detecting areas.
There have already been proposed, in such conventional focus detecting apparatus, technical arts for expanding the focus detecting area or suppressing the generation of color aberration in the peripheral part of the focus detecting area. However the optical configuration proposed in the Japanese Patent Laid-open Application No. 63-243906 has been associated with a major drawback that the further increase in the number of distance measuring points or the expansion of the viewing field is difficult.
Though the presence of a mirror having a converging power in an oblique position behind the main mirror of the single lens reflex camera is effective for expanding the viewing field as explained in the foregoing, it is difficult to effectively apply such re-imaging system composed of a concave face and a convex face in the re-imaging portion of the focus detecting optical system proposed in the U.S. Pat. No. 4,147,417. This is because the color aberration, generated by the oblique positioning of the field lens relative to the optical axis, is extremely large and is not negligible though such re-imaging system is so constructed as not to cause steep refraction of the entering light, in order to suppress the influence of the chromatic aberration.